protocol: drop unused import of zlib

Something weird is happening that breaks pyflakes installed via 'pip
install --user'. I haven't had a chance to finish debugging this, but
this at least fixes the build.

mercurial/hgweb/protocol.py

@@ -8,7 +8,6 b''
 from __future__ import absolute_import
 
 import cgi
-import zlib
 
 from .common import (
     HTTP_OK,